The invention relates to the field of optical fiber communications. More particularly, the present invention relates to the demodulation of optical signals in quadrature for photonic processing and optical data communications.
Quadrature demodulators are used to separate quadrature mixed product signals typically generated during quadrature modulation, using a local oscillator. Quadrature demodulators are also used for separating signals having frequencies above or below the local oscillator reference. The mixing of the signals is usually performed by a pair of modulators modulating a local oscillator signal in quadrature for producing the mixed product signals in quadrature. The mixed product signals have respective frequencies that are typically and equally above and below the oscillator frequency of the local oscillator. The mixed product signals may be for example, IandQ quadrature signals having a quadrature component (Q) and an inphase component (I) for providing communications over a Q channel and an I channel. The quadrature and inphase components have a 90xc2x0 phase shift offset from each other, and are used to modulate a carrier having an RF microwave frequency or an optical frequency. The electrical demodulators are readily implemented at radio frequencies (RF) and up to the microwave frequencies because the 90xc2x0 phase shift between the I channel and Q channel can be accurately maintained at those frequencies. At optical frequencies, the 90xc2x0 phase shifts can be generated using optical hybrids, but the phase shift preceding the optical hybrids cannot be accurately maintained over physically separated optical channels, such as optical fiber channels, that are environmentally sensitive. Optical fibers change in length, diameter and polarization over time due to temperature and stress, introducing variable performance characteristics over the operational life of the optical fiber. Optical wavelengths are very short compared to physical path lengths of interest within an optical demodulator. Consequently, small percentage changes in optical path lengths translate into large optical phase changes providing inaccurate phase shifts during optical signal demodulation. Hence, up to the present time, optical quadrature demodulators have only been considered for free space propagation that does not incur the variable optical performance associated with optical fibers.
Prior optical systems have failed to maintain optical phase coherency between physically separated optical channels that are subjected to unknown and time varying phase perturbations such as those occurring in optical fiber connected photonic processing systems. In a quadrature system, for example, imprecise fiber optical lengths inject unknown phase perturbations such that there is a lose of coherency between the I and Q optical paths, rendering physical fiber optical paths, as guided wave channels, unsuitable for coherent optical demodulation. These and other disadvantages are solved or reduced using the invention.
An object of the invention is to provide optical demodulation of an optical signal input using guided propagation in an optical quadrature demodulator.
An object of the invention is to provide optical demodulation of an optical signal input using guided propagation in an optical quadrature demodulator having close loop control for detecting phase changes within a guided media.
Yet another object of the invention is to provide optical demodulation of an optical signal input using guided propagation in an optical quadrature demodulator for detecting phase changes within a guided media and for maintaining coherent heterodyne demodulation over phase changes in guided media performance.
Still another object of the invention is to provide optical demodulation of an optical signal input using guided propagation in an optical quadrature demodulator having a pilot tone for detecting phase changes within optical fibers and for maintaining coherent homodyne demodulation over changes in optical fiber performance.
The present invention is directed to a guided propagation quadrature demodulator for preferred use in optical receivers for optical data guided communication over an optical link. Guided propagation is preferably realized by optical fibers communicating an optical input signal. The optical fibers have variable phase changes over time. Close loop control maintains coherent demodulation so that the demodulator is effectively insensitive to the variable phase changes of the optical fibers. An optical local oscillator (OLO) is used to provide a large OLO signal and a small pilot tone.
The I and Q pilot signals are used as I and Q input signals during homodyne demodulation. During homodyne demodulation, a pilot tone from the OLO is routed through I and Q channel input optical fibers and then mixed with the large OLO signal also from the OLO for also producing the error component for close loop control. In homodyne demodulation, the error component in the RF domain is used to generate an error signal for phase modulating the OLO signal for coherent demodulation.
The OLO provides an I OLO signal and a 90xc2x0 phase shifted Q OLO signal that are respectively I and Q phased shifted respectively by I and Q phase shifters, and then respectively mixed with the I and Q piloted optical input signals, using homodyne detection, to provide RF quadrature and RF inphase output signals. Optical balanced detectors mix the phase modulated I and Q OLO signal with the I and Q input signal for providing the I and Q error components in the RF domain. An RF local oscillator provides f and 2f frequency references to respective phase sensitive detectors for respectively generating I and Q phase errors between the OLO and the pilot tone or input signal. The I and Q phase errors are respectively added by phase summers to the f and 2f references for providing respective I and Q frequency error signals. The I and Q frequency error signals in I and Q parallel and independent closed loops control the phase modulation of the I and Q OLO signals for coherently aligning the I and Q OLO signals to the I an Q pilot signal or input signals for respective accurate coherent homodyne or heterodyne demodulation.
During either homodyne or heterodyne signal demodulation, the OLO signal is communicated in quadrature with the modulator having a quadrature (Q) channel and an inphase (I) channel, each of which having a phase modulated OLO signal that is mixed for coherent detection of an input signal. During heterodyne demodulation, an optical input signal is routed through I and Q channel fibers for mixing with the large OLO signal for generating an error component for close loop control of the phase modulation of the I and Q OLO signal during coherent demodulation.
As the I and Q channel optical fibers communicate the pilot tone or input signals, the optical fiber performance characteristics over time result in the injecting of unwanted variable phase shifts. These unwanted phase shifts are detected for coherent aligning the I and Q OLO with the I and Q input signal for coherent demodulation. The demodulator advantageously and accurately maintains coherent demodulation of I and Q input signals such that the I and Q RF output signals do not degrade with the changing optical fiber performance user closed loop control. Hence, optical phase coherence is maintained between the physically separated I an Q optical fibers, that are subjected to unknown and time varying phase perturbations, such as those occurring in optical fibers connected to a photonic processing system.
The invention can maintain phase coherence between I and Q channels along separate optical fibers. By injecting a weak optical pilot tone into the input signal, the pilot tone and input signal are exposed to the same environmental perturbations of the optical fibers. The pilot tone signal or input signal effectively tracks the phase perturbation within the optical fiber. The input signal or pilot tone in the I channel and Q channel tracks the phase shifts, as a phase modulated OLO is mixed with the pilot tone for coherent detection. The OLO phase shift with respect to the pilot signal or input signal is controlled by the RF frequency error signal that is proportional to the phase offset generated from phase-balanced detection. The I and Q channels have separate closed loops for separate tracking of the phase shifts along the I and Q channel optical fibers. The phase difference between the input signal in two optical fibers and the OLO is separately tracked by respective close loops. The frequency error signals are generated by phase detection of the error component and the reference. Phase modulation of the OLO signal under closed loop control maintains coherent demodulation in both channels. The I channel balanced detection produces the I channel error component at the RF modulation frequency as the Q channel balanced detection produces the Q channel error component at twice the RF modulation frequency. The I channel frequency error signal is generated at the RF modulation frequency and the Q channel frequency error signal is generated at twice the modulation frequency and in quadrature to the I channel. Hence, the closed loop control generates an error signal in the RF domain for optical phase modulation of the OLO signal for coherent demodulation of the I and Q input signals. The I and Q channels do not suffer the same time delay through the respective I and Q optical fibers before the input signal and OLO are mixed together. Thus, two I and Q channel closed loop controls function independently and serve track the phase perturbations through the I and Q channel optical fibers for accurate heterodyne or homodyne demodulation. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.